1. Field of the Invention
The present invention relates to a device for controlling the operation of a fuel evaporative purge system of an internal combustion engine, and more particularly, to a device included in a system provided with a canister for absorbing and temporarily storing a fuel vapor, a purge passage communicating the canister with an intake passage, and a solenoid valve arranged in the purge passage.
In such a system, when the engine is operating under a predetermined condition, i.e., a purging condition, the device activates the solenoid valve to thereby communicate the canister with the intake passage, whereby the fuel vapor is separated from an absorbent contained in the canister and supplied to combustion chambers of the engine.
2. Description of the Related Art
Among conventional devices for controlling the operation of a fuel evaporative system of an internal combustion engine, Japanese Unexamined Patent Publication No. 62-174557 discloses a device provided with a solenoid valve which is arranged in a purge passage connecting a canister for absorbing fuel vapor with an intake passage for activating the valve at a duty cycle to thereby purge an appropriate amount of the fuel vapor corresponding to an actual driving condition (for example, engine load, engine speed) of the engine.
In the above device, if the current driving condition, which is in a driving range wherein the purging of the system is prohibited (for example, at a low engine load and low engine speed), is changed to a condition which is in a driving range wherein the purging operation can be carried out (for example, at a high engine load and high engine speed), the operation of solenoid valve is controlled so that a value of the duty ratio at which the valve is operated will be gradually increased at a constant rate until reaching a target value determined in accordance with the prevailing driving condition, to thus prevent an abrupt variation of the air-fuel ratio and thereby stabilize an output torque of the engine.
Since the above device employs a method of gradually increasing the duty ratio of the valve at a constant rate, to reach the target duty ratio at the beginning of the purging operation, if a vehicle equipped with the device is abruptly accelerated or decelerated during the process for attaining the target duty ratio, the amount of fuel vapor purged from the canister cannot be rapidly increased in response to abrupt changes of the amount of intake air or the engine load.
Therefore, for example, when such an abrupt change in the intake air amount occurs in an internal combustion engine having an air-fuel ratio feedback control system provided with an air-fuel ratio detecting sensor in an exhaust pipe, to thereby control the air-fuel ratio to a stoichiometric air-fuel ratio, as shown in FIG. 5, a feedback correction coefficient FAF will be changed over a wide range, because the amount of fuel vapor purged is not sufficient to supply a required amount of fuel in response to the change of the intake air amount.
Consequently, the air-fuel ratio of the above engine will be incorrect, and therefore, an output torque of the engine will become unstable.